On black grouse (Tetrao tetrix) leks females prefer dominant, viable males that have managed to acquire relatively central territories. The immunocompetence hypothesis predicts that, because high levels of testosterone are costly to the immune system, male sexual traits that are controlled by testosterone are likely to serve as reliable indicators of male health. Indeed, testosterone concentrations of black grouse males were highly variable, and strongly correlated with male mating success. This is related to the fact that males with high testosterone levels had most central territories. However, the association of testostestorone level with male mating success was not solely related to centrality. Indeed, the particularly strong correlation suggests a possibility for an important role of the immunocompetence hypothesis in this lekking bird species.